There are many types of vehicles designed to travel on several or on just one guide rail. Typically, such vehicles have one or more drive wheels that propel them along the guide rail. To accomplish this, a certain amount of torque has to be applied to the drive wheel or wheels engaged with the rail by a drive mechanism. In this way the state of motion of the vehicle can be controlled, e.g., motion at constant velocity or rapid acceleration as required by the application.
The drive force that is delivered by any drive wheels engaged with a guide rail is limited by traction. Consequently, since acceleration requires a certain amount of drive force and faster acceleration requires more force, the permissible acceleration is limited by traction. In many situations the drive force is applied by one traction wheel while others are provided for stability and control (e.g., idler wheels). Therefore it is usually the friction between the drive wheel and the bearing surface of the rail on which the drive wheel rolls that presents the limiting factor on maximum available drive force.
In a general configuration, for instance in a car, the center of gravity is balanced between the vehicle's wheels. A number of solutions exist to increase the normal contact load on traction wheels in such cases, including foils and springs. In fact, the prior art teaches that these solutions can also be applied in vehicles traveling on guide rails, including monorail vehicles traveling along just one rail.
For example, U.S. Pat. No. 5,069,141 to Ohara et al. discloses an overhead conveyor that provides increased reactive force and traction to a drive wheel on ascending rail sections. The conveyor engages the upper side of the track or rail. Its various means for creating a reactive force are positioned to engage the underside of the track to improve frictional forces during ascendancy. More precisely, the weight of the unit is employed to create the reactional force while guide rollers are resiliently biased by either separate springs or by making the guide rollers themselves resilient. Ohara's teachings are applicable to monorail type conveyors that convey articles along a path defined by the guide rail.
Another solution to monorail vehicles addressing stability and hill climbing capability with the aid of springs can be found in the teachings of U.S. Pat. No. 4,044,688 to Kita. Here a monorail transport apparatus travels while holding the monorail from above and below and uses a driving belt in conjunction with an auxiliary wheel. The apparatus deploys a compression spring to accomplish the intended objectives including increased traveling stability irrespective of the sinuousity of the monorail.
Still other solutions use hydraulics. For example, U.S. Pat. No. 5,372,072 to Hamy teaches a transportation system in which the vehicle is coupled to a track by a bogie whose wheels are mounted on mutually articulated frames. These frames are forcibly urged to pivot with the aid of hydraulic rams. In other words, Hamy teaches to achieve wheel contact load, and consequently maximum driving force, with the aid of certain types of hydraulics.
In contrast to the above references, some prior art solutions teach acting on the wheels of monorail vehicles without the use of springs or hydraulic elements. Rather, they teach to take advantage of the vehicle's own weight. For example, U.S. Pat. No. 3,935,822 to Kaufmann teaches a monorail trolley designed to travel on a monorail and having a truck in which the center of gravity of both the loaded and empty trolley truck is displaced with respect to the points of contact between the rail and the supporting wheel and the counter-wheel. This causes both wheels to engage firmly and adhere to the rail. Kaufmann's design accommodates rapid and easy placement of the truck on the monorail and permits the trolley to move up and down grades. He also teaches adjustments in the placement of the center of gravity without the use of springs or hydraulics.
There are many other prior art teachings that use the center of gravity of a monorail vehicle to achieve their objectives. The reader is referred here to U.S. Pat. Nos. 4,690,064 and 6,321,657 both to Owen as well as U.S. Pat. No. 7,650,843 to Minges and the many additional references cited therein.
Unfortunately, none of the prior art teachings, whether using springs, hydraulic elements or just the placement of the vehicle's center of mass are compatible with large increases in contact load on drive wheels of monorail vehicles that are light, low-cost and yet provide for periods of rapid acceleration along the guide rail as the vehicle transports itself between docking stations. Furthermore, the prior art does not address monorail vehicles that exhibit such desirable features and performance characteristics while being confined to travel along a low-grade (e.g., stock) rail that exhibits a substantial profile variation.